1. Field of the Invention
The present invention relates in general to disk storage systems and more particularly, to a method and apparatus for generating servo information so as to provide gain linearization for positioning the read head of a hard disk drive.
2. Description of the Related Art
Disk drives are magnetic recording devices used for the storage of information. The information is recorded on concentric tracks on either surface of one or more magnetic recording disks. The disks are rotatably mounted to a spin motor and information is accessed by means of read/write heads that are mounted to actuator arms which are rotated by a voice coil motor. The voice coil motor is excited with a current to rotate the actuator and move the heads. The read/write heads must be accurately aligned with the storage tracks on the disk to ensure proper reading and writing of information.
To accurately write and read data, it is desirable to maintain the head on the center of the track. To assist in controlling the position of the head, each sector of the disk typically contains a number of servo bits accurately located relative to the centerline of the track. The raw signals produced by the servo bits are typically demodulated into a position signal which is utilized to determine the position of the head relative to the track, and to move the actuator arm if the head is not located on the track centerline.
Dual element transducers are being increasingly utilized in hard disk drives because they have greater aerial densities than single element transducers. Dual element transducers include a single write element and a separate read element which is constructed from a magneto-resistive material. Such dual element transducers are commonly referred to as magneto-resistive (MR) heads. These MR heads are typically narrow, in the order of half the width of a track.
FIG. 1A is a graph illustrating the variation of the servo burst signals A, B, C, D with respect to the position of the read head of a disk drive under near-ideal conditions. FIG. 1B is a graph illustrating the variation of the difference between servo burst signals (Axe2x88x92B) and (Cxe2x88x92D) with respect to the position of the read head of a disk drive under near-ideal conditions. The variation of the servo burst signals A, B, C, D and the variation of the difference between servo burst signals (Axe2x88x92B) or (Cxe2x88x92D) with respect to the position of the read head are two typical techniques used to provide correlational information between the amplitude of the position error signal and the distance between the head and the center of the track. In the first prior art technique, either of one servo burst signal, A, B, C, or D is used to provide correlational information. In the second prior art technique, two signals, (Axe2x88x92B) or (Cxe2x88x92D), are used to provide correlational information. In both cases, the servo burst signals vary monotonically with the track position of the read head under near-ideal conditions.
Although such monotonic variation permits the servo system to correct the off-track position of the read head, the variation is nonlinear, especially in regions of the servo burst that are more than 25% off the center of the track. Thus, when the servo burst signals are read and demodulated into the position signal for moving the actuator arm, the position signal and thus, the movement of the actuator arm are also nonlinear. This ultimately results in nonlinear movement of the MR head.
Moreover, in most disk drives, the actual variation of the servo burst signals with respect to the track position for an MR head is non-monotonic and non-linear in nature and varies greatly between different heads. FIG. 2A is a graph illustrating the variation of the servo burst signals A, B, C, D with respect to the position of the read head of a disk drive under actual conditions. FIG. 2B is a graph illustrating the variation of the difference between servo burst signals (Axe2x88x92B) and (Cxe2x88x92D) with respect to the position of the read head of a disk drive under actual conditions. This non-linearity is especially pronounced when the MR head is 25% off the center of the track because at that location, the values of the servo burst signals do not vary monotonically with respect to the position of the head.
U.S. Pat. No. 5,781,361 entitled xe2x80x9cMethod and Apparatus for Generating Servo Informationxe2x80x9d filed May 1, 1996 and assigned to the assignee of the present invention, describes a method and apparatus for providing servo information which varies monotonically with respect to the track position of the read head. The provision of such servo information permits accurate alignment of the read head.
However, there is a need in the technology for a method and apparatus for providing servo information which varies linearly with respect to the track position of an MR head. The provision of such servo information facilitates linearization of the servo loop gain, which improves the read throughput performance and consequently, increased product yields.
A method and apparatus for generating servo information used in positioning the read head of a hard disk drive. The apparatus comprises a disk having a plurality of tracks, at least one of which has a data field which includes a plurality of servo bits, each of said servo bits being radially offset from an adjacent servo bit by a predetermined amount. In one embodiment, the predetermined amount is 5% of a width of the track. A servo signal based on the plurality of servo bits is generated and used to position the head relative to a track of the disk.